This invention relates to porous separators which are disposed between the electrode plates of a battery.
Storage batteries include a plurality of electrode plates which are arranged to provide alternating positive and negative electrodes. The separators are made from an insulating porous material and hold battery electrolyte, such as acid, and allow passage of ionic current between the plates.
Battery separators in general must possess certain properties. The separator medium must be resistant to degradation and instability in the environment of the battery, such as degradation by strong acid solutions at ambient and elevated temperatures and strong oxidative attacks. The separator should also be capable of allowing a high degree of ionic movement or should have a low electrical resistance. The separator should also be capable of inhibiting the formation of conductive paths between plates and consequent shorting. This latter problem can arise during battery operation when parts of the battery electrode become dispersed in the electrolyte and precipitate or become deposited in the separator.
Flooded cell lead acid batteries have been in general use for many years. In such batteries, the separators employed typically have a fixed thickness. These type of separators are not highly porous and do not absorb significant amounts of acid. They serve primarily to prevent migration of particles and typically have ribs to physically separate or space the electrodes in the cell.
A recently developed electrochemical cell is commonly referred to as a sealed or valve regulated recombinant design. In certain types of recombinant batteries, the reservoir of electrolyte is completely contained or absorbed by the separator media, and the separator is in full contact with adjacent electrodes and fills the entire space between the electrodes.
Battery separators of the recombinant type must have a degree of empty void volume to permit transport of oxygen gas generated at the positive electrode, during charging or overcharging, to the negative electrode where such gas is reduced. In lead-acid batteries, generated oxygen must pass from the positive electrode through the separator to the surface of the negative electrode, which is damp with sulfuric acid. The oxygen then combines with the lead to form lead oxide, which is in turn converted to lead sulfate and free water.
To achieve the above properties, it is known to employ a mat or felt of borosilicate glass microfibers as the separator media. These separators generally comprise a blend of glass fibers of varying length and diameter. GB patent no. 1,364,283 describes a separator medium made up of fine glass fibers. The fiber mat has a small pore size and provides a very high volume retentivity of electrolyte per unit volume of separator. The capillarity of the mat retains the electrolyte stably within the separator. The mat is designed to be saturated with liquid electrolyte to about 85-95 percent of the available void volume, with the remaining void volume being open to allow gas transfer.
Separators containing submicron glass fibers have several disadvantages which have not been adequately resolved. Health concerns have been expressed about extremely fine fibers of this nature. Glass fiber mats are difficult to process on high speed production equipment due to poor mechanical properties, and they tend to release airborne particles.
Several early proposals were made to use meltblown fibers to make battery separators for conventional flooded cell acid batteries. U.S. Pat. Nos. 3,847,676, 3,972,759 and 4,165,351 all disclose the formation of battery separators from fine meltblown fibers. The fibers are rendered wettable by the addition of internal or external surfactants. In all cases, the mat is permanently compressed, usually by use of heat and pressure, in order to make the pad rigid and to reduce pore size to an acceptable level.
Up to the present time, however, the only material available for use in lead acid batteries of the recombinant type have been mats made of the aforementioned fine glass fibers. Mats made from the meltblown polymers described in the above references are not suitable because of their low porosity, large pore size even when compressed, and inability to completely absorb the acid electrolyte while retaining an empty void volume capable of transmitting gas between electrodes.